1. Field of the Invention
The present invention relates to a fuel supplying apparatus for an internal combustion engine in which an improvement is made in the mechanism for adjusting the pressure of fuel conveyed from a fuel pump to a fuel injector.
2. Description of the Related Art
In a conventional fuel injection system as shown in FIG. 15, fuel is supplied from a fuel tank 1 to a fuel injector 3 by a fuel pump 2, and is injected from the injector 3 into an engine 4. The pressure of fuel conveyed form the fuel pump 2 to the injector 3 is adjusted by a pressure regulator 5 to maintain a predetermined pressure difference with respect to the intake pipe pressure. The conventional system also comprises a return pipe 6 for returning surplus fuel to the fuel tank 1.
Because the predetermined pressure difference between the fuel pressure and the intake pipe pressure is maintained by the pressure regulator 5, the quantity of fuel injected into the engine 4 (hereinafter, referred to as "fuel injection quantity") is proportional to the length of a pulse (injection pulse) applied to the injector 3, as indicated in FIG. 6. Therefore, in high-load engine operation where the fuel injection quantity is increased, the length of injection pulses is increased, as illustrated in FIG. 7B. In addition, because the injection timing of the injector 4 is synchronized with the engine speed, for example, one or two injections per combustion cycle, the length of an injection period decreases as the engine speed increases. Thus, injection intervals become significantly short in high-speed and high-load engine operation.
Normally, if fuel is injected during an intake stroke, so-called "blow-through" occurs during a valve overlap period when intake and exhaust valves are open, that is, fuel injected immediately goes into the exhaust system. To avoid this undesired event, fuel injection should be completed before the intake stroke starts. However, since injection intervals are short in high-speed and high-load engine operation as described above, injection will very likely continue into the intake stroke period (see the shaded portion of FIG. 7B) in such engine operation, and the blow-through of injected fuel will occur, thus resulting in an undesired emission composition having a significantly large amount of unburned gas. If the injector 3 is increased in size to shorten injection pulses in the high-load engine operation, injection pulses in low-load engine operation will become too short. Excessively short injection pulses degrade the fuel injection quantity controllability. As indicated by the dotted line in FIG. 6, if the length of injection pulses applied to the injector 3 becomes less than the lower linearity limit, the fuel injection quantity cannot be reliably controlled.
It is also known that in a case where the engine is restarted after high-load engine operation under high-temperature conditions, the engine startability is deteriorated by the accelerated fuel vaporization (hereinafter, referred to as "hyper-vaporization"). A method to prevent the hyper-vaporization is disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-125984, in which the fuel pressure is increased relative to the pressure in the intake pipe 7 by ceasing introduction of the intake pressure into the pressure regulator 5. However, ceasing introduction of the intake pressure into the pressure regulator can achieve only a limited increase of fuel pressure, resulting in rather insufficient mitigation of hyper-vaporization. Furthermore, this conventional fuel supply system needs to employ a pressure regulator 5 and a return pipe 6 for returning surplus fuel to the fuel tank 1, thus requiring a complicated piping arrangement.